“First and foremost it makes our industry look less legitimate. It’s hard for people outside of the industry to know where the lines are drawn,” she said.

Harris said it’s easy for people to sell on Craigslist, especially for those who get discounts through medical cards.

She said people can go into legal pot shops, purchase weed at a lower price with their medical cards, turn around and sell the pot on Craigslist for a lower price than the recreational shops -- and still make a profit.

“They’re making more money because they don’t pay the taxes, they don’t necessarily have to pay their employees. It’s a completely different business when it comes down to it. It’s just the same product,” she said.

Harris stressed it’s not just people with medical cards. State law allows each resident to grow up to six plants apiece. Harris said some people will grow marijuana at home and sell it online.

“Our narcotics unit does conduct undercover operations where they reach out through ads just like this,” said Doug Schepman, a spokesman with the Denver Police Department.

Schepman said investigators try to shut down as many illegal sales operations as they can by constantly thumbing through the posts on Craigslist.

Most of the posters on Craigslist will try to dodge the law by using words like "donation" instead of payment. Regardless, police said it’s still illegal. Schepman said there is no legal way to sell or marijuana on Craigslist.

“I think the easiest rule of thumb for people to understand is in Colorado there’s no legal market place online for buying and selling marijuana,” Schepman said.

Police said it is legal for someone to give marijuana away for free as long as they’re not accepting money or something for trade in return.

“When you go to the point of saying, ‘I’m going to transfer marijuana to you for a trade or a donation,' [you’re] breaking the law,” Schepman said.